1. Field of the Invention
The present invention relates to a D/A converter circuit, an organic electroluminescent (EL) drive circuit and an organic EL display device which is a self-emissive type display that uses organic materials formed into thin films and that is driven by electric current to generate light. Particularly, the present invention relates to an improvement of a D/A converter circuit constructed with a current mirror circuit capable of obtaining a large analog current even when a power source voltage is low. Further, the present invention relates to an improvement of an organic EL drive circuit having a D/A converter circuit constructed with a current mirror circuit for generating a drive current according to display data and supplying the drive current to a terminal pin of a column line (anode side drive line of an organic EL element) of an organic EL panel so that the D/A converter circuit of the organic EL element drive circuit can generate a drive current having a peak current for initially charging the organic EL element while driving the D/A converter circuit with low voltage, without increasing an area of each of drive circuits provided correspondingly to terminal pins of the organic EL display panel.
2. Description of the Related Art
An organic EL display panel of an organic EL display device for use in a portable telephone set, a PHS, a DVD player, PDA (portable digital assistances), etc., including 396 (132×3) terminal pins for column lines and 162 terminal pins for row lines has been proposed. The number of the terminal pins for column lines and row lines is still increasing.
JP2003-234655A assigned to the assignee of this application discloses an organic EL element drive circuit provided for each column pin of an organic EL display panel, in which, in response to a reference drive current and display data corresponding to the column pin, which is supplied to a D/A converter circuit provided correspondingly to the column pin, a drive current for each of the terminal pins arranged in a column direction or a current on a basis of which the drive current is derived is generated by D/A converting the display data according to the reference drive current.
In order to reduce power consumption, the power source voltage of the D/A converter is restricted to about DC 3V and only the power source voltage of a final output stage current source is set to, for example, DC 15V to DC 20V. The D/A converter responds to the reference drive current to generate the current on which the drive current of the organic EL element is generated and which drives the output stage current source. In this manner, power consumption of the whole drive circuit is restricted. In such case, the drive current for driving the organic EL element, which is supplied to the terminal pin connected to the organic EL element, has a peak portion in an initial drive stage in order to initially charge the organic EL element, which is a capacitive load.
Generation of the peak portion of the drive current is performed by either a circuit portion of an organic EL drive circuit, which is preceding the D/A converter circuit, or a circuit portion of the organic EL drive circuit, which is subsequent to the D/A converter circuit. In JP2003-234655A, such peak current generator circuit is provided between the D/A converter circuit and an output stage current source.
The peak current generator circuits are provided correspondingly to the respective column pins and each requires a large current. When such peak current generator circuit for each column pin is provided between a D/A converter circuit and an output stage current source and the number of column pins is increased, the circuit size of the current drive circuit is increased correspondingly. In order to solve this problem, U.S. patent application Ser. No. 10/360,715 (corresponding to JP2003-308043A) assigned to the assignee of this application discloses a technique in which a peak current generator circuit is added to a D/A converter circuit, which is constructed with a current mirror circuit.
When the peak current generator circuit constructed with the D/A converter circuit is added to the current mirror circuit, it is necessary to increase an output analog current of the D/A converter circuit up to a current value, which is large enough to obtain the peak current. When the D/A converter circuit includes a current mirror circuit constituted with MOS transistors in order to reduce power consumption, it is impossible to obtain large enough analog current in the output side transistor of the current mirror circuit unless a large amount of drive current is supplied to the input side transistor by making the gate-source voltage VGS of the input side transistor of the current mirror circuit larger. However, when the power source voltage of the D/A converter circuit is about DC 3V or lower, the gate-source voltage VGS of the input side transistor is limited by the power source voltage as low as about DC 3V or less and it becomes impossible to generate the large enough peak current. As a result, it becomes difficult to obtain the peak current, which is about 10 times a steady state drive current.
In order to solve such problems, it may be considered to shift a level of the drive voltage of the input side transistor of the current mirror circuit by means of a level shift circuit. In such case, however, the circuit size may be increased as in the case where the peak current generator circuit is provided independently.
On the other hand, when the digital display data is converted into analog signal according to the reference drive current by the current mirror circuit, variation of conversion characteristics of D/A converter circuits corresponding to the respective terminal pins causes the output currents of the column pins to be varied, resulting in luminous unevenness and luminous variation on a display screen of the organic EL display panel.
The luminous unevenness and luminous variation can not be absorbed completely by mere regulation of values of the reference drive currents supplied to the D/A converter circuits. Therefore, a regulator circuit for regulating the analog current obtained by the D/A converter circuit has to be added to the D/A converter circuit. However, since such regulation circuit has to be provided for each of the column pins, the size of the whole circuit becomes large when the number of circuit elements of the regulator circuit is increased, resulting in that it becomes difficult to fabricate the current drive circuit as one IC chip.
In order to solve such problems, U.S. patent application Ser. No. 10/948,237 assigned to the assignee of this application discloses a technique in which a D/A converter circuit is constructed with transistors having large channel lengths to reduce variation of output voltages of the output side transistors of the D/A converter circuits. However, when such transistors having large channel lengths are used, the voltage VGS of the input side transistor of the current mirror circuit must be made large. Therefore, the use of transistors having large channel lengths in the D/A converter circuit has a defect that the power source voltage can not be reduced sufficiently.